PURPOSE: To make easier the centering of couplers on working machine and vehicle sides by a method in which a holder on the working machine side is vertically moved in a rocking manner.

CONSTITUTION: A working machine side holder 86 with a working machine side coupler 54 is supported in a vertically rocking manner by means of a supporter consisting of a rod 92 and a spring 95. By the holder 86, a vehicle side coupler 53 and a working machine side coupler 54 are centered with each other. Under the centered condition, the holder 86 is approached by the holder 56 on vehicle side and both the couplers 53 and 54 on the vehicle and working machine sides are connected with each other.